1. Field of the Invention
The present invention relates to a multi charged particle beam writing apparatus and a multi charged particle beam writing method. For example, the present invention relates to a method of obtaining the high accuracy of multi-beam writing.
2. Description of Related Art
The lithography technique that advances microminiaturization of semiconductor devices is extremely important as being a unique process whereby patterns are formed in the semiconductor manufacturing. In recent years, with high integration of LSI, the line width (critical dimension) required for semiconductor device circuits is decreasing year by year. The electron beam (EB) writing technique, which intrinsically has excellent resolution, is used for writing or “drawing” a pattern on a wafer, etc. with an electron beam.
As an example using the electron beam writing technique, there is a writing apparatus using multiple beams (multi-beams). Since it is possible for a multi-beam writing apparatus to perform irradiation with multiple beams at a time, throughput can be greatly increased compared with the case of writing using a single electron beam. In such a writing apparatus of a multi-beam system, for example, multiple beams are formed by letting an electron beam emitted from an electron gun assembly pass through a mask with a plurality of holes, blanking control is performed for each beam, and each of unblocked beams is reduced by an optical system and deflected by a deflector so as to irradiate a desired position on a target object or “sample” (refer to, e.g., Japanese Patent Application Laid-open (JP-A) No. 2006-261342).
In an electron beam writing apparatus, each shot beam is focused on a target object surface by an objective lens, and then focal correction (dynamic focusing) is dynamically performed during writing in order to deal with convex and concave of the target object surface by using an electrostatic lens, for example. However, when dynamic focusing is performed, rotational variation occurs in a beam image on the target object surface, and moreover, magnification change also occurs. Such problems degrade the accuracy of writing position. In the case of a single beam system, since the number of beams is one, rotational variation and magnification change will occur with respect to one shot, thereby producing a positional error that is not so large in many cases. However, in the case of a multi-beam system, as being different from the single beam system, since the number of beams of one shot which are simultaneously irradiating is a large number, if rotational variation and magnification change occur in the entire multi beams, a writing positional error may the one that cannot be allowed. Therefore, it is required to reduce as much as possible the rotational variation and magnification change of an image due to dynamic focusing. Needless to say, even in the single beam system, such a problem may become serious in the pursuit of further accuracy of writing from now on.